不久前,我遇到了一些代码,它们用mutable关键字标记了一个类的成员变量。据我所知,它只是允许你在const方法中修改变量:

class Foo  
{  
private:  
    mutable bool done_;  
public:  
    void doSomething() const { ...; done_ = true; }  
};

这是唯一的使用这个关键字还是有更多的它比满足眼睛?从那以后,我在一个类中使用了这种技术,将boost::mutex标记为mutable,允许const函数出于线程安全的原因锁定它,但是,说实话,这感觉有点hack。


当前回答

它允许区分按位的const和逻辑的const。逻辑const是指对象没有以通过公共接口可见的方式更改,就像锁定示例一样。另一个例子是一个类,它在第一次请求时计算一个值,并缓存结果。

因为c++11 mutable可以用在lambda上,表示用value捕获的东西是可修改的(默认情况下不是):

int x = 0;
auto f1 = [=]() mutable {x = 42;};  // OK
auto f2 = [=]()         {x = 42;};  // Error: a by-value capture cannot be modified in a non-mutable lambda

其他回答

在为类测试目的创建存根时,mutable关键字非常有用。你可以存根一个const函数,仍然能够增加(可变的)计数器或任何你已经添加到存根的测试功能。这使存根类的接口保持不变。

在某些情况下(比如设计糟糕的迭代器),类需要保留一个计数或其他一些附带的值,这并不真正影响类的主要“状态”。这是我最常看到使用mutable的地方。如果没有mutable,您将被迫牺牲设计的整个const-ness。

对我来说,大多数时候这也像个黑客。在非常非常少的情况下有用。

Mutable主要用于类的实现细节。类的用户不需要知道它,因此他认为“应该”是const的方法可以是。让互斥量可变的例子是一个很好的规范示例。

对于那些对用户来说是逻辑上无状态的东西(因此在公共类的api中应该有“const”getter),但在底层实现(你的.cpp中的代码)中不是无状态的东西,使用“mutable”。

The cases I use it most frequently are lazy initialization of state-less "plain old data" members. Namely, it is ideal in the narrow cases when such members are expensive to either build (processor) or carry around (memory) and many users of the object will never ask for them. In that situation you want lazy construction on the back end for performance, since 90% of the objects built will never need to build them at all, yet you still need to present the correct stateless API for public consumption.

Mutable is used when you have a variable inside the class that is only used within that class to signal things like for example a mutex or a lock. This variable does not change the behaviour of the class, but is necessary in order to implement thread safety of the class itself. Thus if without "mutable", you would not be able to have "const" functions because this variable will need to be changed in all functions that are available to the outside world. Therefore, mutable was introduced in order to make a member variable writable even by a const function.

指定的可变变量通知编译器和读取器 成员变量可以在const中修改,这是安全和预期的吗 成员函数。